In certain thermal printers, a thermal head is modulated (energized) to transfer dye from a dye bearing web (donor, carrier), such as an ink film, to a dye receiving sheet (medium), such as recording paper, to print images in a predetermined, e.g., margin defined, print area thereon, by known heat transfer technique. Typically, the web is interposed between the head and the sheet, and the head is urged against the web and sheet during printing for efficient dye transfer from the web to the sheet.
In each printing cycle, the sheet is advanced to register its print area with the head, whereupon the head is moved from an inactive position to an active position, e.g., from a position spaced from the sheet to a position in urging contact with the web and sheet, and is then energized to effect printing. The web and sheet move in unison at uniform speed past the head during printing. At the end of the cycle, the head is moved to spaced position and the process is repeated for the next cycle.
For line printing, where the head spans the transverse width of the web and sheet print area, in crosswise relation thereto, the web and sheet move longitudinally an increment equal to the printing line height to register them with the head for printing the next line. When printing a continuous longitudinal portion of the sheet, as in forming a graphic design of contiguous printed lines, compared to spaced text lines, the web and sheet move continuously at uniform speed past the head for such line printing.
Typically, the web is wound as a roll on a payout spool for payout, travel past the head, and takeup on a takeup spool. The takeup spool is usually driven by a motor to conduct the web past the head at uniform speed during printing. The sheet is typically fed at the start of a cycle to register its print area with the head, and then travels past the head in unison with the web during printing. This travel in unison is coordinated with the modulation of the head by known technique to effect the printing.
In certain single color thermal printers, the head is modulated to transfer dye from a single color dye bearing web to the sheet to print images on its print area.
In certain multicolor printers, the web has a repeating series of successive dye bearing areas of different colors, e.g., yellow, magenta and cyan, and the sheet is conducted repeatedly past the head to transfer dye from each color area of the series in turn to the same print area. In others, a plurality of heads is arranged in succession, each with a dye bearing web of a single color different from the others, and the sheet is conducted past each head in turn to transfer dye of each color to the same print area.
In some of these printers, a rotating platen drum conducts the sheet past the given head. Typically, in a multicolor printer with a single head having a web of a repeating series of different color dye areas, a single drum reregisters the sheet with the head to print the next color thereon. Typically, in a multicolor printer with a plurality of heads, each having a different color web, either a different drum is used for each head, or the heads are arranged around one drum that conducts the sheet past each head.
Each platen drum is typically driven by a motor coordinated with the associated web takeup spool motor so that the modulation of the head and rotation of both the drum and takeup spool produce a series of pixel elements of correct aspect ratio on the sheet.
For instance, to attain a 1:1 length to height pixel aspect ratio, corresponding to the printing of a line of undistorted square images, e.g., each of a 0.003 inch pixel length and a 0.003 inch pixel height, the head must be modulated (energized) for a modulation time concordant with the rotational (peripheral) speed of both the drum and the web being taken up by the takeup spool. This concordance must be such that the sheet and web are conducted at a proper uniform linear speed for an incremental length of the sheet and of the web, equal to the pixel image height, to travel in unison past the head in a travel time equal to the modulation time.
If the drum is not driven but rather is an idler drum, motor driven conveying rollers move the sheet through a nip between the head and drum. Here, the conveying roller motor is coordinated with the given web takeup spool motor so that the head modulation and rotation of the conveying rollers and takeup spool produce a series of pixel elements of correct aspect ratio on the sheet.
Printing is staggered in those multicolor printers with a plurality of heads arranged in succession, each having a different color web from the others. This is because a sheet moves past each head in turn to transfer dye of each color to the same print area. Thus, in a cycle of a three color printer that prints successively yellow, magenta and cyan in superimposed images on each sheet, the first head prints images from a yellow web on a fresh sheet, while the second head prints images from a magenta web on a sheet already printed with yellow images, and the third head prints images from a cyan web on a sheet already printed with yellow and magenta images.
In the first cycle of a three color (three cycle) run, a first sheet is advanced to and printed at the first head, while the second and third heads are idle. In the second cycle, the first sheet is advanced to and printed at the second head and a second sheet is advanced to and printed at the first head, while the third head is idle. In the third cycle, the first sheet is advanced to and printed at the third head, the second sheet is advanced to and printed at the second head, and a third sheet is advanced to and printed at the first head. The set of three cycles is repeated for each sheet, which leave the third head as completed print products.
At the end of the run, i.e., in the next to last cycle of the last three cycle set, the advancing of a sheet to and printing at the first head are prevented, so that the first head is idle while the already advanced sheets at the second and third heads are printed. In the last cycle, the advancing of a sheet to and printing at the first and second heads are prevented, so that the first and second heads are idle while the sheet previously at the second head is advanced to and printed at the third head.
A problem with this staggered cycle printing of different colors on the same sheet at successive heads is that a web may reach its end of roll or "empty" condition at any random time during a printing cycle. The run must then be aborted and the empty web replaced by a fresh web, before the run, i.e., in sets of three cycles, can be resumed. As a result, the partially printed sheets at the three heads are normally discarded, and a new first cycle is started with a fresh sheet at the first head and none at the second and third heads. This leads to wastage of the discarded sheets and of the web portions used for their partial printing.
It is desirable to prevent this wastage by controlling the printing so as to avoid an end of roll condition of any web during a cycle of such a successive head multiple cycle arrangement.
One way is to provide marks or codes at the terminal ends of the webs for detection to indicate the approach of an end of roll condition of each web. The code of a web at a given head must be spaced from its terminal end a distance sufficient to complete the printing of the sheet at that head and at each preceding head. This entails placing the code at a first distance from the terminal end of the web at the first head, at a longer distance from the terminal end of the web at the second head, and at a still longer distance from the terminal end of the web at the third head.
The first distance must be at least the length of the sheet print area for full printing at the first head of the sheet then at the first head. The second distance must at least twice the first distance for full printing successively at the second head of the sheet then at the second head and the sheet then at the first head. The third distance must be at least three times the first distance for full printing successively at the third head of the sheet then at the third head and those then at the first and second heads.
Also, this limits use of the first distance coded web only at the first head, the second distance coded web only at the second head, and the third distance coded web only at the third head. More important, the only practical way of providing such codes on the webs is to add them to the webs by separate manufacturing steps after web fabrication. This undesirably burdens production costs.
Various thermal printer arrangements are known for single color or multicolor printing of sheets (image receiving members), using dye bearing webs (film or ribbon continous materials). Examples of such arrangements are shown in the following prior art.
U.S. Pat. No. 4,507,667 (Tsuboi) discloses a single head thermal printer with a black ink film as a continuous, single color dye layer web for printing images on paper. During printing, the paper moves past the head via a rotating platen roller, and the film moves past the head as it is fed from a payout roll and increasingly wound on a takeup roll. Complex and costly means decrease progressively the rotational speed of the takeup roll relative to that of the platen roller to compensate for the increasing film radius on the takeup roll, while keeping constant the travel speed of the film and paper. The head forces the film against the paper during printing, creating wrinkles that cause local transfer failure. Complex and costly means remove the wrinkles by a randomly repeated procedure of temporarily releasing the head force, and pulling the film an increment past the head. As the film is used in random increments by this procedure, its rate of use is non-uniform and its end of roll condition cannot be accurately monitored. Thus, the film may reach its end of roll condition during printing, requiring film roll reloading and resulting in wastage of the incompletely printed paper portion.
Commonly assigned U.S. Pat. No. Re. 33,260 (Stephenson, i.e., the inventor herein) discloses a single head thermal printer that sequentially prints different colors from a multicolor web on the same sheet in successive cycles. The web is a light-transmitting carrier having a continuous dye layer of a repeating series of different color dye bearing areas (frames), e.g., yellow, magenta and cyan. On printing one color when a frame and the sheet are registered with the head, the sheet is reregistered with the head and the next frame is registered therewith to print its color. Light emitters and detectors cooperate to identify the colors as the frames move in the printing cycles. The intensity of a given type emitted light is detected by a given detector on passing through the web at a given color frame. The detectors generate signals used by a logic circuit to control cycle operation. This single head printer does not involve staggered printing on the same sheet at successive heads as occurs with a multihead printer.
U.S. Pat. No. 4,863,297 (Fujii) discloses a multihead thermal printer with a plurality of, e.g., three, heads arranged in succession around a platen roller (drum) for sequentially printing different colors, e.g., yellow, magenta and cyan, on the same sheet. The sheet is fed to the platen roller for rotation to each head in turn. Each head has an ink film as a dye bearing web with a continuous dye layer of a color different from the others. Each film has a detectable end of roll mark (code) at a distance from its terminal end different from those of the others concordant with the successive position of the associated head relative to the platen roller. Each mark must be sufficiently ahead of the film end so that on detection the printing at all heads of a sheet then at the first head can be completed while feeding of the next sheet to the first head is prevented. Providing such a mark on each film is complicated and expensive, as it differs in position on the film concordant with that of the associated head around the platen roller, and must be added in a separate manufacturing step after completion of the fabrication of each film. Besides, this coding limits use of each web only at the head for which it is coded.
It is desirable to avoid such additional manufacturing steps for including means to indicate the end of roll condition of a web of a multiweb printer with successively arranged heads, yet provide end of roll responsive control.